Communication over telecommunication networks, in particular voice-handling networks such as the public switched telephone network (PSTN) has fostered an ever-increasing demand for improved speech quality, and for improved reliability of voice-band data transfer and fax. In order to cope with these demands network providers constantly search for means to control and/or eliminate factors, which contribute to interference in a communication channel, such as, for example, echo.
Echo in a communication channel is a phenomenon in which a delayed and usually at least partially distorted replica of an original signal sent from a first terminal (communication device) in the communication channel is reflected back to the first terminal from an echo source at, or on the way to, a second terminal in the communication channel. Echoes conventionally referred to as network or electronic echoes are often a result of an impedance mismatch in a connection of a two-wire telephone line to a four-wire telephone line provided by a hybrid circuit at a switching exchange of the PSTN. Acoustic echoes are generated by acoustic coupling of a phone speaker to the phone microphone Echoes are not only annoying but often superimpose on signals transmitted between over the channel degrading the quality of signals. Electronic and acoustic echoes are discussed below.
Generally, two-wire telephone lines are used to connect subscribers' communication devices to switching exchanges, hereinafter referred to as “exchange(s)”, which route signals from different subscribers to their appropriate destinations. Four-wire telephone lines are typically used to connect exchanges. At the exchanges, the two-wire telephone lines from subscriber communication devices are connected to four-wire telephone lines by means of connectors known as hybrid circuits.
A signal sent by a subscriber, referred to hereinafter as “sender”, to another subscriber, referred to hereinafter as “receiver”, travels from the sender's device through a two-wire telephone line to the sender's local exchange, where it passes through the hybrid circuit into a four-wire telephone line. The signal is then routed through four-wire telephone lines to the local exchange of the receiver. At the receiver's local exchange, the signal passes through another hybrid circuit, which couples the signal to a two-wire telephone line leading to the communication device of the receiver.
Impedances between the two-wire telephone lines and the four-wire telephone lines connected at a hybrid are often mismatched. As a result, a portion of a sender signal received at the hybrid circuit in the receiver's switching exchange follows a path at the hybrid circuit known as an “echo path” that couples the portion into the four-wire telephone line so that it propagates back to the sender as an echo of the sender's own signal. The echo path is characterized by an echo path impulse response function, which “acts” on the sender signal to generate the echo.
Acoustic coupling between a speaker, for example of a receiver's phone or computer, and the microphone in the phone or computer, is usually a source of acoustic echo. A replica, generally modified, of a sender's voice, which is made audible and transmitted by the receiver's speaker, is picked up and retransmitted by the receiver's microphone to the sender generating thereby the acoustic echo. Acoustic echoes are affected by reflections from objects, such as wall, floors furniture and people in the neighborhood of the speaker and microphone. As in the case of network or electronic echo, an echo path impulse response function characterizes acoustic echoes. However, since a neighborhood of a speaker and microphone is subject to change as people and/or objects in the neighborhood move and/or are moved relatively frequently, an acoustic echo path is generally subject to more rapid change than an electronic echo path. As a result, acoustic echoes can be more difficult to deal with than electronic echoes.
Electronic and/or acoustic echoes, in a communication channel generated from a sender signal are often removed and/or reduced by the use of an echo canceller, installed at the receiver end of the communication channel. The canceller is adapted to generate a signal, hereinafter referred to as an “echo copy”, substantially similar to the actual echo of the sender signal. The echo copy is subtracted from signals propagating from the receiver towards the sender by a subtractor module comprised in the canceller. If the echo copy is substantially equal to an actual echo it will substantially cancel the actual echo. A difference between an actual echo and a copy echo is generally referred to as the “residual echo error” and is equal to 0 if the echo is completely cancelled.
An echo copy is typically provided by an adaptive filter, which is comprised in the canceller and which produces the echo copy responsive to a model, hereinafter referred to as a “model echo path function” or “model function”, of the echo path impulse response function. In order to provide the adaptive filter with the model function, the filter “trains” on, or “adapts” to, signals it receives and attempts to produce echo copies of the signals that are substantially replicas of actual electronic and/or acoustic echoes generated responsive to the received signals. Training or adaptation may be performed during a dedicated training session in which only training signals are transmitted to the filter. Training signals and echo path models for testing of speech echo cancellers are described in International Telecommunication Union ITU-T G.168 (August 2004), entitled “Digital Network Echo Cancellers”, incorporated herein by reference. However, generally, training is not performed during or only during dedicated training periods but is an on-line process of adaptation in which the canceller continuously or periodically updates or adjusts itself to attempt to improve its echo canceling performance. Continuous or periodic on-line training—adaptation—is usually performed responsive to signals, e.g. data or voice signals, normally transmitted over the communication channel.
Typically, during training, the filter comprised in an echo canceller receives a portion of a signal received at a receiver, or for example at a hybrid circuit, and generates an echo copy based on a first approximation of the model function. The echo copy is then subtracted from an actual echo generated at the receiver or hybrid responsive to the received signal and a “residual echo error” is obtained, which is fed back into the adaptive filter. The filter uses the residual echo error to provide a new estimation of the model function. The described sequence is repeated in an iterative process until an estimation of the model function satisfies a suitable convergence criterion. A time required for convergence is referred to as “convergence time”.
A problem encountered by cancellers is their response to narrowband “tonal signals”, for example 2100 or 2225 Hz “answer” tones, various call progress, ring back and dial tones used in the PSTN, that are often transmitted over a communication channel. A sinusoidal signal is an example of a narrowband tonal signal. Typically, as depicted in the echo path models shown in ITU-T G.168, echo paths exhibit a relatively wideband frequency response over the voice frequency spectrum. If the canceller trains on and adapts to only tonal signals, the canceller's filter may be adapted to produce a model function that functions properly only for frequencies in the narrow frequency range of the tonal signal. An echo copy may then extend over only a relatively small portion of the frequency range for which echoes are produced. Echo cancellation may therefore be effective substantially only in a range of frequencies spanned by the tonal signals.
A relatively large number of echo cancellers attempt to remedy this problem by freezing or slowing their adaptation process, when they receive tonal signals although this remedy has several disadvantages. For example, if the training is frozen, incorrect handshaking may result due to the superposition of the echo on the tonal signal, which may result in tonal signal distortion, and therefore communication is not established.
U.S. Pat. No. 5,592,548, “System and Method for Avoiding False Convergence in the Presence of Tones in a Time-Domain Echo Cancellation Process”, describes a system for inhibiting false convergence in an echo canceller. The echo canceller comprises an adaptive filter with an adaptation step size controlled to allow the adaptive filter to converge on an input signal.